Beyond CD47: The Next Cancer Immunotherapy Star Target - CD24 Emerges

Q: How do macrophages, as immune cells, normally combat cancer cells?

Macrophages, as a crucial component of the body's innate immune system, possess the ability to recognize, engulf, and eliminate abnormal cells, including cancer cells. In a healthy organism, they detect "abnormal signals" from cancer cells through surface receptors and subsequently launch an attack. This process is akin to having internal "patrols" that continuously potential health threats. However, cancer cells are not passive targets; they have evolved multiple mechanisms to evade macrophage surveillance, with one of the key strategies being the release of "don't eat me" signals.

Q: What is CD47, and what role does it play in cancer immune evasion?

CD47 is a protein widely expressed on the surface of normal cells. By binding to the receptor SIRPα on macrophages, it delivers a "don't eat me" signal, thereby preventing healthy cells from being mistakenly phagocytized. However, cancer cells cunningly exploit this mechanism: they overexpress CD47 protein, disguising themselves as normal cells and making it impossible for macrophages to recognize their danger. This discovery was first revealed by Irving L. Weissman's team at Stanford University School of Medicine, among others, and has driven the development of antibody drugs targeting CD47. For example, clinical trials have shown that the CD47 antibody Hu5F9-G4, when combined with rituximab, has significant efficacy in certain lymphoma patients.

Q: Why does merely blocking CD47 sometimes fail to effectively restore the anti-cancer function of macrophages?

Although CD47 antibodies show promise in some cancer types, clinical studies indicate that not all patients respond to such drugs. Professor Weissman pointed out that this suggests cancer cells may possess other unknown "don't eat me" signaling mechanisms. If cancer cells can persistently grow in a tumor microenvironment densely populated with macrophages, they likely employ multiple molecular pathways to suppress immune attacks. This redundancy mechanism has spurred scientists to continue searching for new immune checkpoint molecules.

Q: How was CD24 discovered, and what role does it play in cancer cells?

In a study published in Nature in July 2020, Weissman's team confirmed that CD24 is another important "don't eat me" signal protein. CD24 (cluster of differentiation 24) is a highly glycosylated membrane protein previously known to inhibit excessive immune responses by interacting with the Siglec-10 receptor on immune cells during inflammation regulation. However, this study found that in various solid tumors (such as ovarian cancer and triple-negative breast cancer), CD24 is significantly overexpressed and binds to Siglec-10 on tumor-associated macrophages, directly inhibiting their phagocytic function.

Q: How do the mechanisms of CD24 and CD47 compare in terms of similarities and differences?

Both CD24 and CD47 belong to the "don't eat me" signal protein family, but their roles in cancer types appear complementary. For instance, blood cancers (such as leukemia) are more sensitive to CD47 blockade, while certain solid tumors (like ovarian cancer) highly rely on CD24 signaling. Experiments have shown that when human cancer cells are co-cultured with macrophages in a dish and antibodies are used to block the CD24-Siglec-10 interaction, the macrophages rapidly engulf the cancer cells. Similarly, in mouse models transplanted with human breast cancer cells, blocking CD24 signaling significantly inhibits tumor growth and extends survival.

Q: What are the clinical prospects for therapies targeting CD24?

The discovery of the CD24-Siglec-10 pathway provides a new target for cancer immunotherapy. Especially for cancer types with limited treatment options and poor prognoses, such as ovarian cancer and triple-negative breast cancer, CD24 inhibitors could represent a breakthrough direction. Professor Weissman stated that such drugs are expected to follow a development path similar to that of CD47 antibodies: first validating safety in preclinical studies, then advancing to human clinical trials. It is important to note that CD24 also has certain functions in normal tissues, so drug design must precisely target the tumor microenvironment to avoid potential side effects.

Q: What does this discovery mean for the future of cancer immunotherapy?

The revelation of CD24 not only broadens our understanding of tumor immune evasion mechanisms but also emphasizes the need for personalized treatment strategies for different cancer types. Combining targeting of multiple "don't eat me" signals (such as CD47 and CD24) may more comprehensively activate macrophage function and improve patient response rates. Additionally, this study reminds the scientific community that there may be more unknown immune checkpoints in the innate immune system worthy of further exploration.

Summary

From CD47 to CD24, scientists have gradually unveiled how cancer cells evade immune attacks through "don't eat me" signals. These discoveries not only deepen our understanding of the tumor microenvironment but also provide direction for developing a new generation of immunotherapies. With advancing research, patients can look forward to more effective treatment options for high-risk cancers in the future.